Some Oxy Anions

We can now consider the effect of the size and shape of the anion on the symmetry and dimensions of the unit cell. For minerals, the anion can vary widely. So far, we have talked only about monatomic (single atom) anions, but many minerals contains polyatomic anions such as carbonate, sulfate, phosphate, vanadate, and, of course, silicates. These are all oxy anions that is, they contain some central atom surrounded by oxygens. It is important to understand that in these anions the oxygens are attached to the central atom by covalent bonds (the bonds may, of course, contain some ionic character). The sulfate, phosphate, vanadate, and nesosilicate ions are tetrahedral in shape. For example, the snlfate ion contains a sulfur at the center of a tetrahedron, as shown below (remember the entire species has a charge of 2-, which is not shown in this structural representation)  

Can you predict whether each of the sulfur-oxygen bonds is primarily covalent or considerably ionic  

Primarily covalent. The electro-negativity of oxygen is 3.5 and the value for sulfur, which is immediately below oxygen in the periodic table, is 2.5 (see Appendix 2). Thus, the difference is 1.0, which suggests that the bonds are mainly covalent. By comparison, the difference between the electronegativities of oxygen and sodium is 2.6. 

How much charge do each of the oxygens of the sulfate ion carry  

How does the ionic character of the Si—O bond compare with that of the S—O bond, and how much charge resides on each oxygen  

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